After using a 3.5 inch Nextion display for building a thermostat, I quickly envisioned using these displays with my latest version of a digital clock. The Nextion display is a leap forward in LCD touch displays. The built-in microprocessor makes building a professional looking touch display interface quite easy and takes a lot of overhead off the microprocessor connected to the display.
If you aren’t familiar with the Nextion displays, you should refer to Mike Holler’s excellent series of articles on these in previous issues of Nuts & Volts. The learning curve for these displays is a little steep and these articles are a great help. I haven’t used the Nextion library in my projects but rather have sent and received direct commands to the Nextion displays through the serial interface.
Nextion displays come in a variety of sizes with different capabilities and features. My digital clocks used the 3.5, 5, and 7 inch sizes. The 3.5 and 5 inch models used are the Nextion basic model while the seven inch is the intelligent model. The Nextion web page at https://nextion.tech describes the different models on the Products tab. All the different sizes I used run from the same circuit and printed circuit board (PCB) described in this article; only the software differs.
The 3.5 and 5 inch sizes have the same features:
The seven inch version has additional features:
The main components on the PCB are:
Miscellaneous parts include: 1,000 µF 6.3 volt electrolytic capacitor, 0.1 µF capacitors (four), header pins, two- or three-pin terminal blocks (two), 5.5 x 2.1 mm jack, five volt/one amp wall wart type power supply with mating plug, various screws, nuts, wire, solder, header pins, two-pin slide on header jumper, 30 cm/four-pin header jumper, project box, and 40-pin IC socket cut in half longitudinally for the ESP32. Figure 1 shows the schematic for this project.
FIGURE 1. Nextion digital clock schematic.
Figure 2 shows the completed circuit on a PCB.
FIGURE 2. Assembled circuit on a PCB.
A PCB is a great convenience when assembling this circuit, but it’s certainly possible to use a breadboard and point-to-point wiring. An ExpressPCB layout for this circuit is included in the downloads. The Express PCB software can be downloaded from https://www.expresspcb.com. Pads are included for either 0.9 or 1.0 inch wide ESP32 development boards. The pads for connecting a micro SD card board are positioned under the ESP32, and are laid out in the correct order to match my micro SD card board.
Check to make sure the micro SD card board you use has the same pin layout. One end of a four inch/six-pin header pin jumper was cut off and the wire ends soldered into the micro SD card pads on the PCB. The optional micro SD card board can then be plugged into the opposite end.
If a micro SD card is not installed, the micro SD card board should be unplugged. Additional pads were included in this PCB design for additions or changes that are not used in the circuit described here.
A four-pin/12 inch long header jumper is used to connect the BME280 temperature, humidity, and pressure board to a four-pin header on the PCB. This keeps the temperature and humidity sensor from being heated by the PCB components. There’s a slot in the back cover of the project box that was 3D printed to bring the four-pin header jumper out.
A four-pin header is on the PCB for connecting the Nextion display. The Rx line on the Nextion display goes to the Tx (pin 17) on the ESP32 and Tx on the Nextion display to Rx (pin 16). The Nextion displays come with a wire connector that has mating connectors for header pins. The DS3231 RTC board has four pads provided at the opposite end of the board from the header pins. A four-pin header is soldered into these pins and then the long ends of the header pins are soldered into the PCB. The socket for the battery on this board should face up. The battery isn’t needed as the time is obtained from the Internet every minute.
The PAM8403 audio amplifier board needs header pins soldered into it. After these are in place, the long ends of the header pins are soldered into the PCB. Note that only the left channel of the amplifier is used. The right channel has breakout pads on the PCB for optional use. The volume potentiometer on this board has a built-in on-off switch which must be switched on.
I found a small 80 x 40 mm/eight ohm speaker on eBay and it worked well for my small project enclosure. Any small eight ohm speaker can be used and wired directly onto the PCB. The pads for the speaker were sized for miniature terminal blocks.
The procedure I used for writing software for the clock functions and communicating with the Nextion display was to first program a Nextion display page (Figure 3) using the Nextion Editor and then add code for the ESP32 in the Arduino IDE (integrated development environment) to utilize the Nextion display page.
FIGURE 3. Nextion display page 0.
The Nextion Editor is available at https://nextion.tech/nextion-editor.
The first page of the Nextion display, page 0, has a background image and four textboxes numbered t0 to t3 and is laid out bottom to top. Textbox 0, t0 in the Nextion program is at the bottom of the page and was made as large as possible because it displays the current time. The three textboxes above the time are used to display the city name of the home location, date, day of the week, sunrise, sunset, and the local temperature and humidity from the BME280.
A string containing the time is sent through the serial interface to the Nextion display once per second to t0 with the following code:
// Write the time to the Nextion
tempstr = String(bhour) + “:”;
if (bminute < 10) tempstr.concat(“0”);
if (last_sec < 10) tempstr.concat(“0”);
sendThis = “page0.t0.txt=\””; // Build the part of the string that we know
sendThis.concat(tempstr); // Add the variable we want to send
writeString(sendThis); // Use a function to write the message
// character by character to the Nextion
// because mySerial.write(sendThis) gives you
// an error due to a datatype mismatch
A string is created using the hour, minute, and second variables and is then added to the string page0.t0.txt”. The closing quote marks are added. So, what we’re sending is page0.t0.txt=”12:34:56” where in our program we use the \” to define a quote mark which is inside the quote marks that define the string. This is a standard format for sending a string to a textbox in the Nextion display.
Note that no spaces are used, and we must define what page the textbox is in, its name such as t0, the .txt qualifier, and then the string itself inside quotes. The following code sends the home location name to the textbox t3 at the top of page 0:
sendThis = “page0.t3.txt=\””;
Each textbox needs a font associated with it; the font number is selected in the Attribute window in the lower right of the Nextion Editor after clicking on a specific textbox. Fonts must be created with the Font Generator (Figure 4) which is found in the Tools menu.
FIGURE 4. The Font Generator.
A selection is made from the list of fonts available on your computer running the Nextion Editor. Its height, or size, is also selected and there’s a checkbox for making it bold.
Lastly, a Font Name must be created before clicking Generate Font. A Save As window will then appear and again a Font Name must be chosen. Click Yes when the Add the Generated Font window comes up.
Your available fonts will appear in the Fonts window by clicking on the Fonts tab at the very bottom left of the Nextion Editor. Once fonts are created, they can be selected for any textbox on any page.
The 3.5 inch Nextion clock has the following pages:
The Menu page (Figure 5) is accessed by touching the textbox containing the time on page 0.
FIGURE 5. Page 1, the Menu page.
From the menu page, other pages can be accessed. Touching textbox t2 on page 0 will speak the time and some weather details.
The Menu page has three dual-state buttons at the top with small textboxes to the right of each one. These act like on-off buttons to define which state a setting is in. They jump to the weather page each minute or not, show Fahrenheit or Celsius for temperature (also changes in wind velocity), and use only the home location or scan between nine different city locations.
The state of a dual-state button is shown in the small textbox to the right. The Home Weather button jumps to page 2, where the home location can be chosen. The Weather Page button jumps to page 3, the weather page. The Picture Frame button jumps to page 4, which displays a series of images that have been added through the Nextion Editor. The Colors button jumps to page 5, where colors can be selected for the textboxes on page 0.
Lastly, there’s a brightness slider to reduce the brightness of the display which is a useful addition for people who don’t want a bright light when used in the bedroom.
Notice here that page 0 of the Nextion display was used by sending differing text to the appropriate textboxes. The use of page 1 is quite different. The dual-state buttons send a Component ID to the ESP32 microcontroller, telling it which button was pressed. This is programmed in the Nextion Editor by checking the Send Component ID box located in the Event window.
When the Component ID is received by the ESP32, the current state is echoed back as a single character to the Nextion display and shown in the small textboxes to the right of the dual-state buttons. The Component ID is received in the mySerialEvent() function and then is passed to the get_Nextion_command() function The remaining five buttons and the brightness slider are all programmed in the Nextion Editor. Their actions only affect things in the Nextion display so the ESP32 doesn’t need to deal with these actions.
Pressing the Home button jumps to page 0. To program this into the Nextion display, the Home button is selected and the Nextion command “page 0” (without the quotes) is entered in the Events window of the Nextion Editor after selecting the Touch Release Event button. For the Home Weather, Weather Page, Picture Frame, and Colors buttons, the Nextion command for the Touch Release Event is page 2, page 3, page 4, and page 5, respectively. With the Brightness slider, h0, we finally get into writing a small piece of code for the Nextion display.
After selecting the h0 slider, click on the Move Event button in the Events window. In the window below, the following was typed in:
As the slider is moved, the dim system variable that controls the brightness of the Nextion display is changed. The value of h0 can range from 0 to 100, but 20 was chosen as the darkest allowed. This system variable is documented in the Instruction Set page which is accessed from the Help tab of the Nextion Editor.
It should be noted here that the majority of buttons on page 1 create actions solely contained within the Nextion display itself, without intervention of the ESP32 microcontroller. This is one of the great advantages of the Nextion displays. The built-in Nextion display microcontroller can take care of many of the desired changes we want in the Nextion display.
The Home Selection page (Figure 6) is page 2 and contains nine buttons with the names of various locations where I have family.
FIGURE 6. Page 2, the Home Selection page.
The ESP32 software defines a default home location but this can be changed in page 2.
When one of the buttons is pressed, the Component ID of that button is sent to the ESP32. So, what does the Component ID actually look like when sent to the ESP32? The easiest way to see this is to use the Debug tool in the Nextion Editor.
When the Tierra Verde button (Figure 7) is pressed, the following bytes are sent out the serial port to the ESP32: 0x65 0x02 0x01 0x00 0xFF 0xFF 0xFF.
FIGURE 7. Debug simulator after the Tierra Verde, FL button is pressed.
The 0x65 designates a touch event; the 0x02 designates the page number; 0x01 is the Component ID; 0x00 is the event (0x00 is released and 0x01 is pressed), followed by the three 0xFF bytes which terminate all commands.
I often change the buttons on this page to match the interest of the person for whom the digital clock is built. Because of this, the Component IDs of the buttons are not always in order or have gaps in the numbering.
Using the Debug Tool is a convenient way to capture the appropriate list. With some components, there’s a second list of values. For example, the state of a dual-state button is sent in the second list of bytes.
Each second the program checks to see if a string of characters has been sent from the Nextion display. If so, the characters are stored in an array called bstring and then the get_Nextion_command() function is called.
Included in this function is a line like:
if ((bstring == 0x65) && (bstring == 0x02) && (bstring == 0x01) && (bstring == 0x00)) myHome = 0;
This if statement captures the fact that the Tierra Verde, FL button has been pressed and assigns the home location to the myHome variable.
The Weather page (Figure 8) simply contains five large textboxes to display details of the weather at the current home location.
FIGURE 8. Page 3, the Weather page.
Its only function is to receive text strings: t0 receives the city name of the current home location; t1 receives a brief description of conditions; t2 receives the temperature; t3 receives the humidity; and t4 receives the wind speed and direction.
A touch on the screen between the textboxes or at the bottom will return to page 0. Note that a fairly dark image was used for the background which works well with white text.
Page 4 shown in Figure 9 contains a photo frame instead of a digital clock function.
FIGURE 9. Page 4, a photo frame.
The memory available allows over 40 pictures to be displayed. All pictures used with a 3.5 inch Nextion display (which are full frame) should measure 480 x 320 pixels.
My pictures consist of the usual family, myself at travel locations, some scenery, and a few birds. The .HMI and .TFT files created by the Nextion Editor (and available in the downloads) will contain my pictures.
To substitute your own, first resize your pictures to 480 x 320 pixels. In the Nextion Editor, pictures are imported in the lower left window. If the heading doesn’t show Picture, then click on the Picture tab at the bottom of the window. Pictures are added with the plus symbol and deleted with the minus symbol (Figure 10).
FIGURE 10. The Picture Selection window.
This page has a timer and a variable which are selected from the Toolbox at the upper left. These components will not appear on the page but do show up at the bottom of the Display page. Settings for these components are selected after you click on one and then review the Attribute window on the lower right.
For the Timer component tm0, I set the time to 10000, which is the time in milliseconds. For the variable va0, I left the default value as 0 and set the Variable type as Number. In the Timer Event window, I added the following:
Every time the Timer counts down to zero milliseconds, this code is run. The variable va0 is increased by one. Then, if the variable is 48, it’s changed to 0 (we have 48 pictures numbered 0 to 47). Lastly, the pic (picture) number on page 4 is assigned the va0 value, and the next picture will appear.
Page 5 shown in Figure 11 has two variables attached: va0 which stores the color number 0-5 for textbox0 on page 0 that displays the time every second; and va1 which stores the color number 0-5 for the remaining three textboxes on page 0.
FIGURE 11. Page 5, the Select Text Color page.
The following code is entered into the Touch Release Window for b0, the white button:
Similar code is assigned to each of the other five color buttons in their Touch Release Events windows. The only difference in the code will be the color numbers assigned to the text and background colors. Sixteen-bit color numbers are used: five bits for red; six bits for green; and five bits for blue.
After the if statement, the textboxes on page 5 are also assigned the white color to demonstrate its appearance. The dual-state button bt0 has additional code. If the dual-state button is in state 0 (not pressed), the color buttons will assign color to the t0 time textbox on page 0.
If pressed, the color will be assigned to the other three textboxes on page 0:
if(bt0.val==0) // Time
t1.txt=”Select Color for Time”
t0.txt=”Temp and Date”
t1.txt=”Select Color for Temp and Date”
It should be noted here that all this code is handled by the Nextion microcontroller, so the ESP32 doesn’t have to use any of its processing time to deal with these colors.
Figure 12 is a basic flowchart of the ESP32 software and is a typical loop structure that checks things as follows:
FIGURE 12. Basic flowchart of ESP32 software.
The Network Time Protocol servers at pool.ntp.org are used to retrieve the time from the Internet. Details on this service are available at https://www.ntp.org. The weather is retrieved from the open Weather API. Details on this service can be found at https://openweathermap.org/api.
Along with this service, you’ll need the city IDs to tell the Weather API which city weather you’re requesting.
The city lists containing the city IDs can be found at http://bulk.openweathermap.org/sample/. These lists are in .gz compressed format. After you decompress a list, use a program like 7-Zip where you can search for a city name. Be sure to check the latitude and longitude coordinates as many cities have the same name.
The major functions in the ESP32 program are:
The entire digital clock program can be found in the downloads.
While I have made a digital clock using the five inch Nextion display, it’s basically the same as the 3.5 inch model but using the larger display.
The seven inch Nextion display I purchased was from the Intelligent series, which has additional features. The seven inch displays are 800 x 480 pixels in size.
The Menu page for the seven inch model is shown in Figure 13 and reflects its additional features.
FIGURE 13. The Menu page for the seven inch Nextion digital clock.
There are buttons for the stock market display, an alarm, and temperature graph.
The Waveform component was used to graph the indoor and outdoor temperatures over the course of a day, reading these values every two minutes.
The layout of the Temperature Graph page is shown in Figure 14.
FIGURE 14. The Temperature Graph page with a Waveform component.
The Waveform component is sized to 722 x 258 pixels allowing for one pixel borders. A reading of the temperatures is done every two minutes; therefore, there are 720 points to be plotted each day.
The temperature value needs to be normalized to range between 0 and 255, based on the scale chosen. The default scale is 50-90 degrees Fahrenheit. A point is plotted on the Waveform component with the following statements:
sendThis = “add 1,0,”;
The string “add 1,0,201” will add a point 201 pixels high to Waveform component 1, channel 0. The temp_graph variable contains the normalized value of the temperature. The temperature range of the graph can be changed using the Graph Menu page, and this change needs to be reflected in the temp_graph values when computed. The 40 degree spread works well for most locations.
The Graph Menu page is shown in Figure 15
FIGURE 15. The Graph Menu page.
This page allows a change in the temperature range as well as selecting the home location.
Adding stock market quotes adds a new layer of complexity. Stock quotes are obtained from the IEX Cloud API. See https://iexcloud.io/docs/api for details. I chose the DOW 30 list of companies for the stock prices I wanted to display. The Stock Market page (page 5) is very similar to the Weather page and contains five scrolling textboxes and one regular textbox.
Because there are a lot of Internet requests to obtain stock quotes, the second core of the ESP32 is set up to continually request stock quotes when the stock market feature is turned on.
Each minute, five of the 30 DOW stocks will be displayed, with the next five the following minute until all 30 are displayed.
To delegate this task to the second core of the ESP32, a Taskhandle is created at the top of the program after the #define statements:
Then, in the setup() function, we create a structure to pin a task to a specific core in the ESP32:
Task0code, // Function to implement the task
“Task0”, // Name of the task
7000, // Stack size in words
NULL, // Task input parameter
1, // Priority of the task
&Task0, // Task handle
0); // Core where the task should run
The Task0code(void * parameter) function is then created and placed after the loop() function:
// Read stock prices using second CPU
void Task0code(void * parameter)
// Task here must never end or return
// Get stock quotes from Internet
for (int j = 0; j < 30; j++)
for (int i = 0; i < 5; i++)
stock_temp[j] = getStocksFromInternet(stock_sym[j]);
if (!(stock_temp[j].indexOf(“,,”) > 0))
tstring = parse_stock(stock_temp[j], stock_sym[j]);
Serial.print(“the_stock = “);
if (the_stock == 30)
dow = total_price / 0.14748071991788; // This value must be kept current for DOW
sendThis = “page5.t0.txt=\””;
sendThis.concat(“DOW Ind. “);
Serial.print(“DOW = “);
the_stock = 0;
total_price = 0.0;
There’s a built-in alarm function which uses the speaking feature when the alarm goes off. On the Alarm Menu page (Figure 16), AM/PM is selected and then the time is entered using the keypad.
FIGURE 16. The Set Alarm Menu page.
The time is stored after pressing the Enter key. The alarm function is turned on or off with the Alarm button.
The Text Color Selection menu shown in Figure 17 is similar to that of the 3.5 inch Nextion digital clock.
FIGURE 17. The Text Color Selection menu.
Digital clocks made with Nextion displays are a great design. The hardware needed is reduced compared with an LED display, which requires either hardware drivers like TTL 7447s or a scanning structure that creates overhead for the microcontroller.
The Nextion display provides numerous components that enhance the appearance of the display and images are available for backgrounds or use with components.
Because of the sophistication of the display, the amount of software that needs to be written to exploit this increases. The pages that appear in the display itself must be programmed with the Nextion Editor. The attached microcontroller also needs additional code to exploit the Nextion display
There are blocks of code to deal with Internet access including reading the time, weather, stock prices, or other data from the Internet. Features like plotting a temperature graph add to the complexity.
The end result (Figure 18) is a display that is much more attractive and informative. NV
FIGURE 18. The finished seven inch Nextion display clock.
What’s in the zip?
Entire Digital Clock Program
.HMI and .TFT Files